Water-soluble acidic polyesters and thermosetting water-based coatings containing them

ABSTRACT

Water-soluble acidic polyesters are prepared by reacting a hydroxy polycarboxylic acid, such as citric acid, with an alkylene oxide, such as propylene oxide, an alkylene glycol, or styrene oxide and polyesterifying to an acid number of about theoretical to about 25% above theoretical. A thermosetting coating comprises the acidic polyester and a conventional aminoplast in an aqueous vehicle. When the coating is applied to a substrate, such as label stock, and is thermoset, a durable high gloss varnish is produced.

This is a division of application Ser. No. 442,945, filed Feb. 15, 1974,now U.S. Pat. No. 3,903,051.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is concerned with novel acidic polyesters and withwater-based coatings containing them and an aminoplast.

2. Description of the Prior Art

In U.S. Pat. No. 3,046,252 there is described a polyester having arelatively low acid number and hydroxyl number, which is produced by thecondensation reaction of benzene tricarboxylic acids with an alkyleneoxide or an alkylene glycol. Coatings are formed with these polyestersand an aminoplast in vehicles conventionally used for alkyd resins, suchas aromatic hydrocarbons. If a water-soluble baking resin is desired,these polyesters must be converted to salts by reaction with an alkalineagent.

The acidic polyester resins of the present invention of relatively highacid numbers and hydroxyl numbers are readily soluble in aqueousvehicles without resort to salt-forming. Insofar as is now known, acidpolyester resins of this type have not been described heretofore.

SUMMARY OF THE INVENTION

This invention provides an acidic polyester produced by polyesterifyingan aliphatic hydroxy polycarboxylic acid and a lower alkylene oxide,alkylene glycol or styrene oxide, using a ratio of hydroxyl (oxide)equivalents to carboxyl equivalents of 5:3 to 2:3, preferably 4:3, at atemperature between about 65°C. and about 150°C. and in the presence ofan alkaline catalyst, until the acid number is at least abouttheoretical to about 25% above theoretical.

The invention also provides a thermosetting coating comprising saidacidic polyester and an aminoplast in an aqueous vehicle; and bakedcoatings obtained therefrom.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The acid component used in the acidic polyesters of this invention ishydroxy polycarboxylic acid. Although citric acid is preferred and usedprimarily to demonstrate the invention, other hydroxy polycarboxylicacids are contemplated such as tartaric acid, malic acid, and α,β-dihydroxytricarballylic acid.

The preferred alkylene oxide, which is used mostly for illustrationherein, is propylene oxide. It is contemplated that alkylene oxides,alkylene glycols, or styrene oxide can be used, however. The alkyleneoxides contain from 2 to 4 carbon atoms. These oxides are ethyleneoxide, propylene oxide, and butylene oxide. The alkylene glycols containfrom 2 to 6 carbon atoms and can be simple glycols or ether glycols.These glycols include ethylene glycol, propylene glycol, 1,4-butanediol,1,3-hexanediol, diethylene glycol, dipropylene glycol, and triethyleneglycol.

The hydroxy polycarboxylic acid and the alkylene oxide, alkylene glycol,or styrene oxide are reacted in amounts less than the stoichiometricamount needed for full esterification of all the carboxylic groups. Ingeneral, the ratio of hydroxyl (oxide) equivalents to carboxylequivalents will be 5:3 to 2:3 and preferably 4:3. Generally, inpractice, however, a slight molar excess of oxide or glycol is used,usually an excess of about 0.05 to 0.5 mole. In some cases, slightlyless than the amount to esterify all but one carboxyl group may be used.

The polyesterification reaction is carried out in the presence of analkaline catalyst. As catalysts there may be employed inorganic ororganic bases such as alkali metal hydroxide, calcium oxide, sodiumamide; alkyl and phenyl ammonium hydroxide, such as trimethylaminehydroxide; secondary amines such as diethylamine, dibutylamine,piperidine; and tertiary amines such as trimethylamine, triethanolamineand pyridine. Catalytic amounts are used, usually about 0.1% to about 5%based on the weight of acid charged. In general, the addition andpolyesterification reaction is carried out at temperatures between about65°C. and about 150°C. In practice using an alkylene oxide or styreneoxide, its addition to the hydroxy polycarboxylic acid is carried out inthe lower range of temperature, in the order of about 65°-90°C. Thepolyesterification is carried out in the higher portion of the range, inthe order of about 110°-150°C. The time for the addition andpolyesterification reaction will, of course, depend upon the temperatureused and is generally between about 3 hours and about 10 hours. Thepolyesterification is complete when the reaction product attains an acidnumber of about theoretical to about 25% above theoretical. In thepreferred case of propylene oxide and citric acid the acid number willbe about 180-225.

After the desired acid number is attained, the reaction mixture issubjected to distillation under reduced pressure to remove low molecularweight polymer. This distillation is generally carried out at atemperature of between about 210°F., (99°C.) and about 302°F. (150°C.)under a vacuum of about 20 to about 30 inches of mercury.

The resultant acidic polyester is a water soluble material which isdissolved in water to form a clear solution at between about 70% andabout 80% solids.

COATING COMPOSITIONS

The acidic polyester can be thermoset using a conventional aminoplastcross-linking agent. Such agents are well known in the art. There can beused any of the thermosetting alkylated aminoplast resins, such as theurea-aldehyde resins, the melamine aldehyde resins, thedicyandiamidealdehyde resins and other aminoplast-aldehyde resins suchas those triazine resins produced by the reaction of an aldehyde withformoguanamine, ammeline, 2-chloro-4,6-diamino-1,3,5-triazine,2-phenyl-p-oxy-4,6-diamino-1,3,5-triazine,6-methyl-2,4-diamino-1,3,5-triazine; 2,4,6-trihydrazine-1,3,5-triazine,and 2,4,6-triethyl - triamino-1,3,5-triazine. The mono-, di-, ortriaralkyl or mono-, di, or tri-aryl melamines, for instance2,4,6-triphenyltriamino-1,3,5-triazine are preferred. As aldehydes usedto react with the amino compound to form the resinous material, one mayuse such aldehydes as formaldehyde, acetaldehyde, crotonic aldehyde,acrolein, or compounds which engender aldehydes, such ashexamethylenetetramine, paraldehyde, paraformaldehyde, and the like.

It is preferred to use an aminoplast that is water soluble. In general,an alcohol, such as isopropanol or ethanol is added to a solution ofaminoplast in water.

Other well known adjuvants may be added, such as flow control agents andwaxes. A preferred flow control agent is sodium dioctyl sulfosuccinate,but others utilizable include sodium dihexyl sulfosuccinate, sodiumdiamyl sulfosuccinate, isopropyl naphthalene sulfosuccinate, andsorbitan monolaurate, monopalmitate, or monooleate. Waxes, if used, areadded as slurries or emulsions of petroleum (paraffin) wax, naturalwaxes such as montan wax, beeswax, and carnauba wax, or synthetic waxessuch as polyethylene wax.

To prepare the final coating composition the acidic polyester solutionis admixed with the aminoplast mixture. These components will usually beblended using between about 60 and about 70 parts by weight of acidicpolyester solution to between about 40 and about 30 parts by weightaminoplast mixture. Other ratios can be used, bearing in mind, however,that higher aminoplast concentrations tend to increase brittleness ofthe final baked finish and that lower aminoplast concentrations tend toincrease bake temperature or bake time, or both. The final productshould have a solids content of between about 60 and about 80 wt.%.

These coating compositions can be applied to the desired substrate assuch or, in some cases, it may be diluted as, for example, about 20/1with isopropanol or water. After application the coating is baked atabout 3 seconds to about 5 minutes at between about 250°F. and about450°F. A preferred bake is for about 10 seconds at 350°F. Because ofsome end uses, however, this bake may be impractical but can be reducedto, for example, 10 seconds at 275°F. by replacing the water andisopropanol with industrial grade ethyl alcohol.

Suitable substrates are paper, metallic foil and can stock metals(approximately 10 mil stock). The coating can be applied by roll coat,flow coat or gravure coat.

EXAMPLE I

Into a reaction vessel equipped with a heat source, reflux condenser,charging port and an agitator were charged 978 grams citric acid, 60grams propylene oxide and 16 grams benzyl trimethyl ammonium hydroxide.Agitation was started and the reactants were heated to 175°F. (79.4°C.).While maintaining the temperature of 170°-180°F. (76.7°-82.2°C) 546grams propylene oxide was slowly added being certain that the additionrate was such that no propylene oxide was lost to distillation orincomplete condensation in the reflux condenser. About 3 hours wasrequired for this addition. After the propylene oxide addition wascomplete, the temperature was raised to 300°F. (148.9°C.) and held(1.5-2.0 hours) until an acid number of 180-190 was obtained. Thereaction mixture was then subjected to the distillation under a vacuumof 24-26 inches of mercury for 1 hour to remove lower molecular weightfragments, maintaining the temperature above 210°F (98.9°C.). Then,while continuing cooling and agitation, 400 g. water was added. Theresultant product was a clear solution of 76-80% solids having a weightper gallon of 10.0-10.5 pounds and a viscosity of 490-500 cps. The finalacid number was 180-190.

EXAMPLE II

In a mixing vessel was stirred 564 g. of alkylated melamine resin (Cymel7273-7), 257 g. deionized water and 123 g. anhydrous isopropanol.Stirring was continued until the components were thoroughly blended andthen 7 g. sodium dioctyl sulfosuccinate (Aerosol OT) were added andstirring was continued until it was dissolved. Finally, 49 g. montan waxslurry in water at 30% solids (Jonwax 25) were added and stirring wascontinued until uniformity was obtained.

EXAMPLE III

There was stirred together 570g. of the product of Example I and 430 g.of the product of Example II. When thoroughly blended, the mixture wasallowed to stand overnight before using. The final product had a resinsolids of 66% in a water-alcohol mixture, a viscosity of 55-65 secondsNo. 2 Zahn Cup at 80°F. and a weight per gallon of 9.75-9.85 pounds.

For use as a paper label varnish this product was thinned 20/1 withisopropanol. It was then applied to a paper label surface at a filmweight of 3.0-3.5 mg/sq.in. and baked 10 seconds at 350°F. (176.7°C.). Ahigh gloss thermoset varnish was produced which had good appearance overboth paper and inked areas.

In order to eliminate the overnight aging mentioned in Example III aportion of the alkylated melamine-formaldehyde resin is removed fromExample II and incorporated into Example I. Also to permit production inequipment having power limitations, a portion of the deionized water canbe added first to dissolve citric acid rather than having it present asa dry material during the first part of the resin preparation. Thesemodifications are shown in the following example.

EXAMPLE IV

To a stainless steel reactor equipped with a heating and cooling source,a reflux condenser and a vacuum system there was charged 77 lbs.deionized water. Agitation was commenced and the water was heated at205°F. (96.1°C.). There was gradually added 429 lbs. of citric acidwhile maintaining agitation and keeping the temperature at least above150°F. (65.6°C.). Then, 7 lbs. benzyl trimethyl ammonium hydroxide wasadded followed by the addition of 266 lbs. propylene oxide over a 4 hourperiod, allowing the temperature to rise to a maximum of 190°F.(87.8°C.). At completion of the propylene oxide addition the temperaturewas increased to 300°F. (148.9°C.) and held until an acid number of180-190 was obtained. Then distillation under a vacuum of 24-26 inchesof mercury was carried out for 1 hour, allowing the temperature to driftto a minimum of 210°F. (98.9°C.). Then, 176 lbs. deionized water and 45lbs. of alkylated melamine resin (Cymel 7273-7) were added and thecontents of the reaction vessel were heated to 250°F. (121.1°C.) andheld for 1 hour. The resulting solution was cooled to 120°F (48.9°C.)and filtered. The final product had a weight per gallon of 10.25 lbs.,total solids of 80% and a 70 second No. 2 Zahn Cup viscosity.

EXAMPLE V

The aminoplast component was made according to Example II, except thatthe weights of the ingredients were 530 g. alkylated melamine resin(Cymel 7273-7), 276 g. deionized water, 132 g. anhydrous propanol, 8 g.sodium dioctyl sulfosuccinate (Aerosol OT) and 53 g. montan wax slurryin water at 30% solids (Jonwax 25).

EXAMPLE VI

60 parts of the product of Example IV was blended with 40 parts of theproduct of Example V. This yielded a finish ready for immediate use,having properties the same as those of the coating of Example III. Thiscoating was applied to label stock with the results as obtained inExample III using a 10 seconds bake at 350°F. (176.7°C.). The bake wasreduced to 10 seconds at 275°F. (135°C.) by replacing the water ofExample IV and the water and isopropanol of Example V with industrialgrade ethyl alcohol.

EXAMPLE VII

Using the equipment described in Example I, there were charged 827g. ofdipropylene glycol and 576g. citric acid. Under agitation, the reactionmixture was heated to 150°C. and held at that temperature until an acidnumber of 130-135 was attained (theoretical= about 123) and atheoretical water removal of 108g. This required about 4 hours. When theproper acid number was reached, reaction was stopped by adding 155 g.water. This produced a water-soluble product of 90% solids that could beblended with an aminoplast and baked, as described hereinbefore toproduce a baked finish.

EXAMPLE VIII

Other combinations of hydroxy polycarboxylic acids and alkylene oxidesor styrene oxide are reacted substantially as described in Example I inan amount to react in a ratio of hydroxyl equivalents to carboxyequivalents of 4:3. The combinations and theoretical acid number are setforth in the following Table. The resultant products are blended with anaminoplast and baked, as described hereinbefore, to produce a bakedfinish.

                  TABLE                                                           ______________________________________                                                                    Theoretical                                       Acid             Oxide      Acid No.                                          ______________________________________                                        Malic            Ethylene   194                                               Malic            Propylene  177                                               Malic            Butylene   163                                               Malic            Styrene    127                                               Tartaric         Ethylene   179                                               Tartaric         Propylene  165                                               Tartaric         Butylene   152                                               Tartaric         Styrene    120                                               α,β-Dihydroxycarballylic                                                            Ethylene   190                                               α,β-Dihydroxycarballylic                                                            Propylene  173                                               α,β-Dihydroxycarballylic                                                            Butylene   159                                               α,β-Dihydroxycarballylic                                                            Styrene    125                                               Citric           Ethylene   200                                               Citric           Propylene  182                                               Citric           Butylene   167                                               Citric           Styrene    130                                               ______________________________________                                    

In the Table, the theoretical acid number is based upon reacting 3 acidequivalents with 4 oxide equivalents. This is the basic combiningproportion to produce the polyester products of this invention that arecombined with an aminoplast to produce a baking resin, in accordancewith this invention. As indicated hereinbefore, the desired degree ofpolyesterification is achieved by continuing esterification until theacid number is between the theoretical acid number and about 25% abovethe theoretical acid number for the particular combination of aliphatichydroxy polycarboxylic acid and alkylene oxide, alkylene glycol, orstyrene oxide used. In general, depending on the reactant combination,the acid number will range from the lowest of 120 (tartaric acid-styreneoxide) to 25% above the highest acid number of 200 (citric acid-ethyleneoxide) or 250, i.e., a range of about 120-250.

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationsmay be resorted to, without departing from the spirit and scope of thisinvention, as those skilled in the art will readily understand. Suchmodifications and variations are considered to be within the purview andscope of the appended claims.

What is claimed is:
 1. A thermosetting coating composition comprising awater-soluble, up to 80 per cent solids, acidic polyester produced bypolyesterifying an aliphatic hydroxy polycarboxylic acid selected fromthe group consisting of citric acid, tartaric acid, malic acid, and α,β-dihydroxytricarballylic acid with a compound selected from the groupconsisting of alkylene oxides having from 2 to 4 carbon atoms, alkyleneglycols having from 2 to 6 carbon atoms, and styrene oxide, using aratio of hydroxyl equivalents to carboxyl equivalents of 5:3 to 2:3,until the acid number is 120 - 250, depending on the reactantcombination, and an aminoplast in an aqueous vehicle.
 2. Thethermosetting coating composition of claim 1, wherein thepolyesterification is carried out in the presence of an alkalinecatalyst, said hydroxy polycarboxylic acid is citric acid, said alkyleneoxide is propylene oxide, and said acid number is about 180 -
 190. 3.The thermosetting coating composition of claim 2, wherein saidaminoplast is a melamine resin.
 4. The thermosetting coating compositionof claim 1, wherein the polyesterification is carried out in thepresence of an alkaline catalyst, hydroxy polycarboxylic acid is citricacid and said alkylene glycol is dipropylene glycol, and said acidnumber is about 130 -
 135. 5. The thermosetting coating composition ofclaim 4, wherein said aminoplast is a melamine resin.